The present invention relates to an automatic document feeder (ADF) for use with an electrophotographic copier, digital copier or similar image recorder for selectively feeding ordinary documents in the form of separate sheets and a continuous document in the form of computer form (CF) paper.
An ADF is extensively used with the above-described kind of image recorder for automatically feeding a document to a glass platen of the image recorder while preventing it from jamming the path or from being damaged, then stopping it on the glass plate, and then discharging it after an image printed thereon has been scanned. Documents usable with the ADF include ordinary documents in the form of separate sheets and elongate documents such as CF paper. Generally, CF paper has a number of sprocket holes formed through a marginal area thereof. Specifically, the sprocket holes are positoned one after another in an intended direction of paper feed to mesh with the teeth of a sprocket which drives the CF paper. A sensor is located on the transport path of the CF paper to sense the sprocket holes, so that the transport of the paper may be controlled in response to the output of the sensor. An ADF having a capability for transporting such CF paper to the glass platen is disclosed in Japanese Patent Laid-open Publication No. 72455/1984. A drawback with a prior art ADF having such a capability is that the document transport control cannot be readily implemented for each of different kinds of documents. Moreover, it is difficult to accurately position a document in a predetermined position of a glass platen for imagewise exposure. Especially, sequentially locating consecutive pages provided on CF paper in the particular position on the platen is extremely difficult.
CF paper with a carbon is a special kind of CF paper and has, in addition to ordinary sprocket holes, some cuts intervening between nearby sprocket holes. Such cuts are adapted to fasten the CF paper and carbon temporarily to each other. An ADF capable of transporting CF paper with a carbon to the glass platen is taught in Japanese Patent Laid-Open Publication No. 72456/1984 by way of example. The ADF disclosed in this Laid-Open Publication adopts an extremely complicated system for discriminating the above-mentioned cuts from the sprocket holes. Specifically, the system is such that the number of sprocket holes are divided into consecutive zones and sensed by a sensor zone by zone. Nevertheless, this prior art ADF is not entirely free from the fear that the cuts intervening between nearby sprocket holes are erroneously determined to be the sprocket holes.